Process for stretching cellulose acetate filaments



Patented June 21, 1949 PROCESS FOR STRETCHING CELLULOSE ACETATE FILAMENTS Percy Frederick Combe Sowter, Spondon, near Derby, England, assignor, by mesne assignments, to Celanese Corporation of America, a

corporation of Delaware No Drawing. Application March 20, 1945, Serial N 0. 583,840. In Great Britain March 29, 1944 2 Claims.

This invention is concerned with the manufacture of artificial filaments of cellulose acetate or other organic derivative of cellulose of low denier by a stretching operation.

It is well known to produce filaments of low denier by stretching already-spun artificial silk of cellulose acetate or the like. These low denier filaments are more particularly prized became of their high tenacity. For example, yarns containing filaments made of cellulose acetate ma be stretched to 5 or or even times their original length in solutions of organic solvents, e. g. an aqueous solution of dioxane. A similar result may be obtained by carrying out the stretching in a medium of Wet steam or hot water under pressure. This stretching of the filaments is attended by an increase in their tenacity and by an orientation of the molecules of cellulose acetate as determined by X-ray diffraction photographs. It is also attended by a very substantial reduction in the extensibility of the filaments. For example, the ordinary extension of to of cellulose acetate filaments may be reduced to 5 or 6%, and moreover the major part of this decrease in extensibility occurs in the early stages of stretching, before the tenacity has risen by more than a small amount. More specifically, it usually occurs in the first 100% stretch. For many purposes such a decrease in extensibility is undesirable, but it appears to be an inevitable result of orientation of the molecules and of increase in tenacity,

In experimenting with the stretching of cellulose acetate and similar filaments I have encountered a type of stretching which appears to involve an entirely new phenomenon. For instance, in stretching cellulose acetate filaments in a mixture of of ethyl acetate and 10% of toluene by volume I have obtained a product which has substantially the same tenacity and extension as the starting material, and yet due to the stretch the filament denier is substantially decreased. It therefore appears that this particular type of stretching has not involved orientation of the cellulose acetate molecules, which, as far as I am aware, has always characterised previous stretching processes. I call this new type of stretching process a slip stretch to differentiate it from the gripping stretch which is associated with a substantial decrease in extension and increase in orientation and tenacity. It appears essential that in order to achieve this slip stretch the softening power of the softening agent shall be very near to that necessary to produce actual solution of the cellulose acetate.

2 Generally the most useful concentrations of the active substances in the softening agents are between the concentration which actually dissolves the materials or destroys their structure and a concentration about 2% by volume below this concentration.

Other agents besides ethyl acetate and toluene may be used, for example ethyl acetate and carbon tetrachloride, acetone and benzene and the like. Probably the degree of softening which is necessary to produce this new slip stretch is higher than that necessary to produce the well known gripping stretch. At least I have found that an important consideration in producing the slip stretch is to soften to such a degree that the tension necessary to stretch is very low. To produce a gripping stretch, tensions of the order of 2 grams per denier are quite common. Tensions somewhat below this value of 2 grams still serve to produce the gripping type of stretch. Ethyl acetate in toluene will. if the softening power is adjusted to give a stretching tension of the order of 1.5-2 grams per 100 denier, give the gripping type of stretch, and the same applies to the other stretching agents mentioned above. However, with concentrations such that the stretching tension is only of the order of .5-.1 gram per 100 denier, a slip stretch is readily obtainable.

With the above agents it is comparatively easy to adjust the conditions so that the stretching tension is maintained at a low value. With some other agents, for instance methylene chloride in benzene, the range of conditions to give slip stretch is so narrow that it is difiicult to maintain the conditions within the necessary limits, and hence the result appears to be partly gripping stretch and partly slip stretch. However, it is probable that with a very accurate control of the temperature, soaking time and concentration, conditions could be maintained within the narrow limits necessary to produce slip stretch. Similarly, where water and alcohols are contained in the stretching agents, for example as diluents, the range of tolerances seems to be even narrower, and I prefer to employ stretching agents which do not contain either water, alcohols or other hydroxyl-containing compounds.

Generally it may be said that slip stretch can be recognised as taking place when the extension of'the yarn is not reduced by more than about 25%. This new phenomenon of slip stretch having been described, it is relatively easy to ascertain which liquids will readily give the slip stretch and which liquids require such accurate control over the conditions that it is difficult to carry out slip stretching in a practical way.

As appears from the above description the softening agents employed are usually mixtures of organic solvents .or swelling .agents for the materials with-organic non-solvents. Softening liquids which are volatile organic liquids having boiling points of the order of 100 C. or less, preferably considerably lower, for example below 80 C., are preferable to liquids-of higher boiling points such as glycol mono-acetate, methyl-glycol mono-acetate and diacetoneealcohol. .Hydrophile softening liquids, e. g. dioxane '.and methylene ethylene ether and particularlyesters and ketones such as ethyl acetate and acetone, appear to be more suitable than hydrophobe liquids such as methylene and ethylenedichlorides. On the other hand. the most satisfactory inert liquids are usually hydrophobe organic liquids, particularly hydrocarbons and halogenated hydrocarbons such as benzene,-toluene, ithe xylenes .and carbon tetrachloride. Suitable softening agents .are a mixture of acetone and benzene containing 37 -to 41% of-acetone-atz20 to 25=C ,ia- 90/10 ethyl acetate/toluene mixturerat 30C. and a 95/5 ethyl acetate/carbon tetrachloride mixture at 30 -.C. All the aboveproportions are :b volume.

The degreetof softeningof cellulose derivative materials produced by-a softening-agent depends upon its temperature, generall-yincreasing as the temperature trises and' theattainment of the .required degree iof softeningfor the :purposeof the present invention depends not-only :on the usaof a suitablesofteningagent but also on its use at a suitable temperature. For "example, r a mixture ofa softening liquid and anlinert liquid in. such concentrations that at atmospheric temperature they lorma softening .agent in which cellulose acetate'yarn can be stretched to-give a product of high tenacity and=lw--extensibility,. may, if used at a temperature of 30 or 40 C.,.give the type of stretch characteristic ofetheprocess of the present invention.

In carrying outthe present process thematerials are. preferably treated withtasofteningagent consisting of a hydrophile organic softeningliquid and aninert: hydrophobe-organic liquid. The treatment maybe effected by spraying the materials but preferably they are immersedin-a bath of softening agent until theyareuniformly softened and stretching-can beeffected with l. a low stretching tension. 'With liquid softeningiagents at lovr.temperatures, e..-g. -20-or 30 (3., .this may requirea treatment-of-from 30 seconds to 5minutesoreven more, according to the particular softening agent .employed and the nature of the cellulose derivative-materials. .For instance, the higher the filament denier the longerisrequired for the filaments to-be thoroughly impregnated with the liquid. The timerequiredfor softening maybe-decreased-by thoroughly drying the materials before treatingsthem with the softening agent,-and the .time which is required for the materials to be -in contact with the final softening agent, i.-e. thesoftening agent in contact with the materials during stretching, before they reach asuitable condition for stretching-may also be reduced .by :pre-treating X them with amixture of the .samesofteningiliquid and inertliquid as used in thefinal softening agent butcontaining a slightly:lower-concentration of the softening liquid, e. g. 1 tor 3% lower. Alternatively softening agents containing :the same constituents in the same proportionszmay he used both :for the pretreatment rand-also.forxthafinahsoftening :treatremain without stretching between the different stretching stages. The materials may be stretched inhanks in a softening agent or during their travel from bobbins through a bath containing the softening agent to take-up bobbins. Alternatively they may be sprayed with the agent .prior to and during the stretching. After the stretching operation is complete, a liquid softening agent may be removed from the materials either by evaporation or by washing, preferably with the inert liquid used in the softening agent. If .an'evaporative method is employed itisimportant that the softening liquid present-in \the softening agent should be more volatile than the inert liquid so that-the concentration of thBzSOfteni'ng liquid in the agent presenton thematerials-does 'not increase during drying, otherwise partial solution of the materials may take place and their physical properties may be detrimentally affected.

During removal of the softening agentfromthe materials it is desirable to :allow acertain amount of shrinkage totake placein order .to obtainthe bestresults as regards extensibility. Thedegree of shrinkage required increases with the degree of stretch effected: .for instance, withastretch of 200% to 300% .of the originallength of the yarn a shrinkage of about v5.to l5 of such length is satisfactory, whereas with a stretch .of 500% to-550% -asmuch as 30% shrinkage may be-desirable.

The process ofthe presentinvention is particularly valuable. in the treatment of filaments made of acetone-soluble cellulose acetate having an acetyl value of 52-54;% calculated as acetic acid, whether of medium viscosity, e. g. 10 or 20, or of higher-viscosity e. g. 40, 80, 150 or more. These viscosities are determined by comparing therate of flow of a 6% solution of the acetate in acetone at 20 -C. with that of glycerine, taken as .100. The process ishowever also applicable .to the stretching of filaments and similar materials made of other cellulose acetates, e. g. a tri-acetate or'an acetate of 50%,'45% or less acetyl value, and of other cellulose derivatives, e. gacellulose acetate-propionate,cellulose butyrate and other lower fatty acid esters, cellulose acetate-stearate, ethyl cellulose and ethyl cellulose. acetate.

The following examples aregiven to illustrate the invention:

Example 1 Yarnmade of acetone-soluble cellulose acetate of about 54%.acetyl value and 20 viscosity .was immersed in .hank form in a mixture consisting of ethyl acetate and 10% toluene .by volume at 30 C. for 30 seconds. 'It was then stretched gradually, -.tension being applied intermittently until .astret'ch .of hadbeen effected. The yarnwas then rinsed .undertension with toluene andallowed to dry .in the absence of tension. During drying a shrinkage equal to about 5% of the original length of the yarn occurred. .After drying .the yarn .obtained had substantially the same. .tenacityandextension .as the original yarn.

Example 2 Hanks of the acetone-soluble cellulose acetate yarn used in Example 1 were stretched as described in this example, using a softening agent consisting of 95% ethyl acetate and 5% carbon tetrachloride by volume at 30 C. and using carbon tetrachloride alone for washing, and similar results Were obtained.

Example 3 Yarn made of acetone-soluble cellulose acetate of 50 viscosity and about 54% acetyl value was stretched during its passage through a 20 foot bath containing a mixture of acetone and benzene in proportions of 41 of acetone and 59% of benzene by volume, the temperature being 20 C. The rate of travel of the yarn was adjusted so that the stretching tension was about 0.3 grm. per 100 denier and the yarn was stretched to six times its original length. After stretching the yarn was allowed to shrink by 30% of its original length and taken up on swifts. The yarn obtained had an extensibility of about 20%.

Having described my invention, what I desire to secure by Letters Patent is:

1. Process for reducing the denier Without substantially affecting the tenacity and extensibility of artificial filaments having a basis of an acetone-soluble cellulose acetate and having an extensibility of at least 20%, which comprises softening the filaments by means of a mixture containing acetone and benzene at 20 to 25 C. until they become capable of being stretched to at least twist their original length by a tension of 0.1 to 0.5 gms./100 denier, gradually stretching the softened filaments to at least twice their original length, washing the stretched filaments with benzene and drying them, and allowing them to shrink during at least part of the Washing and drying operation.

2. Process for reducing the denier without substantially afiecting the tenacity and extensibility of artificial filaments having a basis of an acetone-soluble cellulose acetate and having an extensibility of at least 20 which comprises softening the filaments by means of a mixture containing 37 to 41% of acetone and 63 to 59% of 5 benzene at 20 to 25 C. until they become capable of being stretched to at least twice their original length by a tension of 0.1 to 0.5 gms./100 denier, gradually stretching the softened filaments to at least twice their original length, washing the stretched filaments with benzene and drying them, and allowing them to shrink during at least part of the washing and drying operation.

PERCY FREDERICK COMBE SOWTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,004,274 Dreyfus June 11, 1935 2,034,713 Dreyfus Mar. 24, 1936 2,047,195 Dreyfus July 14, 1936 r 2,051,831 Dreyfus Aug. 25, 1936 2,058,422 Dickie et al. Oct. 27, 1936 2,066,492 Sowter Jan. 5, 1937 2,081,172 Dreyfus May 25, 1937 2,081,173 Dreyfus May 25, 1937 2,112,237 Dreyfus Mar. 29, 1938 50 2,239,780 Fikentscher et a1. Apr. 29, 1941 2,277,163 Sowt-er et al Mar. 24, 1942 FOREIGN PATENTS Number Country Date 323,790 Great Britain Jan. 16, 1930 OTHER REFERENCES Freund et al., Internal Structure of Synthetic Fibres and its Influence on their Physical Properties, Rayon Text. M0,, 1942, 23, pages 515-517, 605-606.

Certificate of Correction Patent No. 2,473,615

PERCY FREDERICK COMBE SOWTER It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 34, for the word twist read twice;

June 21, 1949 and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 20th day of December, A. D. 1949.

THOMAS F. MURPHY,

t Assistant Oommz'moner of Patents. 

